Communication between a host station and several remote stations over a limited number of lines is an area which has been a subject of numerous patents and proposed architectures. The key areas addressed by these approaches include the number and function of lines which interconnect the host station with the remote stations, the powering of the remote stations, the addressing of the remote stations by the host station, the requesting of remote station transmissions back to the host station, and the interfacing requirements between the remote stations and the interconnecting lines.
The cost of such systems depends upon the particular architecture chosen. The particular architecture chosen also governs whether a system is usable in a particular environment. In environments where access is limited, where power is not available, where the conditions are hazardous or where there are specific requirements on wiring entering or leaving the area, the state of the art architecture does not provide a satisfactory solution.
Typical of a generalized communications bus are the RS232 and RS485 bus standards. These architectures employ separate power, address and digital data lines. The series 9000 product, manufactured by Palomar Technology International of Carlsbad, Calif., employs a local area net among local monitoring units, based upon the RS485 standard. The Sensorbus Product, manufactured by Transensory of Fremont, Calif.; the Digital Transmitters Series R2000 product, manufactured by Rochester Instrument Systems of Rochester, N.Y.; and the Octapak product, manufactured by Action Instruments of San Diego, Calif.; employ an RS232 standard interconnection scheme among remote units.
Two line systems exchange power, addresses and data over a common pair of lines. As such, there are a variety of techniques used to permit the simultaneous use of the lines for such different purposes. U.S. Pat. Nos. 4,555,695 and 4,727,359, to Machida et al. and Yuchi et al., respectively, disclose two wire systems pertinent to the present invention. However, among other things, the two wire systems disclosed utilize data transmission and addressing schemes which significantly limit the manner in which data can be obtained from the remote stations and the information content of any data transmitted. Other two-wire systems include those disclosed in U.S. Pat. No. 4,200,862 to Campbell et al., which is directed to a carrier current approach; in U.S. Pat. No. 4,441,107, to Chaborski, which provides addresses on a different line from analog data, and in which power is supplied independently at the site of the remote stations; and in U.S. Pat. No. 4,573,041 to Kitagawa, U.S. Pat. No. 4,203,096 to Farley, and U.S. Pat. No. 4,613,848, to Watkins, which apply power, address and digital data on the same pair of lines. In a product called the Sensor Highway, manufactured by Westinghouse Energy Systems, Nuclear Technology Systems Division, Pittsburgh, Pa., AC power and analog data are provided on the same pair of wires. In a product called the S-Net, manufactured by Solatron, Schlumberger, of Hampshire, England, a 2-wire cable is used to carry power and bi-directional data between remote isolated measurement pods. In a product called the Transpak, manufactured by Action Instruments of San Diego, Calif., two wires are used to provide power to a remote sensor and a signal path for signals from the remote sensor.
Four wire systems employ two pairs of wires with each pair carrying different combinations of power, addressing and signal information. U.S. Pat. No. 4,331,952, to Galvin et al. discloses a four wire system having one wire each for "signal", "power", and "carrier" functions, which interconnects adapters which handle redundant sensors. U.S. Pat. No. 4,290,055 to Furney et al. employs a four wire system in which data pulses from remote detector stations are returned on one wire, power is supplied on a separate wire, and an interrogation pulse is passed from detector station to detector station on a third wire which interconnects the detector stations in series.
U.S. Pat. No. 4,032,916 to Galvin et al., discloses a five-wire system which does not appear to employ the addressing of individual transceiver units.
Other references which represent the state of the art include Japanese Patent No. 59-160350 to Yasuda; Japanese Patent No. 59-172857 to Tadachi; Japanese Patent No. 56-40345 to Oka; U.S. Pat. No. 4,683,531 to Kelch; U.S. Pat. No. 4,667,193 to Cotie; U.S. Pat. No. 4,654,654 to Butler; U.S. Pat. No. 4,595,921 to Wang; U.S. Pat. No. 3,755,781 to Haas; U.S. Pat. No. 4,369,436, to Lautzenheiser; U.S. Pat. No. 4,529,971, to James; U.S. Pat. No. 4,575,711, to Suzuki et al.; and U.S. Pat. No. 4,628,308, to Robert.